The present invention relates to a ballasting method for an oil tanker. An oil tanker is operated with a full load of oil on one way from a loading base to an unloading base at which the oil is refined and with a minimum load or without load on the returning way from the refining base to the loading base. It is well known that on the return voyage, the ship becomes top-heavy, leading to a danger of capsize. In order to overcome the above problem, it is common practice to "ballast" the ship by filling the tanks with seawater on the return way. However, when an oil tanker is ballasted with seawater, it is necessary to exhaust seawater from the tank before loading crude oil. Since exhausted seawater may contain some oil component, there is a possibility of seawater contamination causing a problem of pollution. Such activity is prohibited by various rules thereby preventing the discharge of ballasting water directly to sea. Therefore, in order to exhaust ballasting water, the ship must be moved to a particular base capable of processing the contaminated water and then to the oil loading base. If the tanker has been designed to have separate compartments for separate loading of crude oil and seawater the above problem may be eliminated. However, with such tanker design the amount of crude oil to be loaded will be greatly reduced.
In order to resolve these problems various designs of tanker compartments have been proposed. For example, Japanese Patent Public Disclosure No. 48-29184 discloses a tanker structure in which ballasting water and oil are separately introduced and discharged without reducing the amount of crude oil to be loaded by using a elastic partition wall of oil-resistant and chemically stable material for each compartment for partitioning into a ballast water section and an oil section. FIGS. 1 and 2 show the structure disclosed by the aforementioned prior art publication in which FIG. 1 is a cross sectional side view of a compartment of a tanker and FIG. 2 shows a cross sectional front view of the compartment in FIG. 1. The compartment 1 is defined by a deck plate 2, a bottom wall 3 and side walls 4 and 4', into which oil and ballasting seawater are introduced. A peripheral portion of a partition membrane 5 made of oil-resilient and chemically stable material is sealingly clamped by clamping members 6 provided along lines A-B, B-C, C-D and D-A as in FIG. 2, to divide the compartment 1 into two liquid tight sections a and b. In this construction, when oil is introduced via a proper inlet into the chamber a the partition membrane 5 is deformed toward the side wall 4, allowing oil to occupy a space of the chamber a. On the other hand, when ballasting seawater is to be introduced into the compartment, the partition membrane is deformed backwardly to the side wall 4' so that the chamber b is filled with seawater. In this manner, the compartment 1 can be filled alternatively with crude oil and ballasting water. However, since the current tankers are extremely large in size, both the compartment and partition membrane become correspondingly large.
For this reason, it has been found that when either chamber a or b is to be filled under a full or empty condition of the compartment 1 the partition membrane 5 is not deformed uniformly, causing high pressure to be locally applied on the membrane. It has been further found that even if it is deformed uniformly, a very large tensile force is applied to the membrane due to a pressure as high as 0.7-1.0 Kg/cm.sup.2 per water level of 10 m.
For this reason the material strength of the membrane is high and expensive.